Data networks are generally constructed using a number of edge switches, also called edge nodes, that are connected to local data sources and data sinks. An edge node consists of a source node and a sink node that may share a common controller. The edge nodes are interconnected by a network core that switches traffic between edge nodes. The network core may be distributed and it may include several geographically distributed core nodes. The network core may also be agile, being able to reconfigure to follow fluctuating traffic demands.
Links interconnecting the edge nodes and the core nodes support communications paths for transferring data between edge nodes. Each of the communications paths has a predetermined granularity defined as an integer multiple of a capacity unit. A capacity unit is the minimum capacity, in bits/s, that can be assigned to a traffic stream. Paths of uniform granularity have been traditionally used in data networks.
The performance, efficiency and scalability of a data network depend heavily on a property of the nodes in the network called “degree” (nodal degree) and on a property of the network, which is directly related to the nodal degree, called “network diameter”. The degree of a specific node is a measure of the number of nodes to which the specific node directly connects. The diameter of a network is a measure of the maximum number of hops (links) in the shortest path between any two nodes. The higher the nodal degree, the smaller the network diameter. A small network diameter generally yields both high performance and high efficiency. On the other hand, for a given nodal degree, scalability generally increases with the network diameter, but to the detriment of network efficiency. It is therefore advantageous to increase the nodal degree to the highest limit that technology permits.
The number of sink nodes to which the source node can send traffic, without the traffic having to be switched at an intermediate edge node, is directly related to the nodal degree of the source node. The nodal degree of a source node is also related to the number of output ports supported by the source node and the speed of switching by the source node. In particular, a fine switching granularity (requiring relatively fast switching) provides a greater nodal degree than a coarse switching granularity. An electronic edge node is inherently fast switching, thereby enabling the division of the capacity of the output ports into relatively small units. To match this capability of the source node, an optical core node is required to be fast switching.
In one known scheme of optical-fiber link capacity division, the links that connect nodes are divided into channels, where each channel is associated with a wavelength. The nodal degree of a source node in this scheme is limited by the number of channels emanating from the source node, which is typically significantly smaller than the number of edge nodes in the network.
In another scheme of link capacity division, links or channels are divided according to a system of time sharing. Time sharing enables fine switching granularity and, hence, a high nodal degree. Schemes based on effective time sharing typically require: (a) that the edge nodes be time-locked to the core nodes; (b) that all nodes be fast switching; and (c) that a path between two edge nodes traverse a single optical core node. A node X is said to be time-locked to a node Y if, at any instant of time, the reading of a time-counter at node X equals the sum of a reading of an identical time-counter at node Y and the propagation time from node X to node Y, where the time-counters at nodes X and Y have the same period and the propagation delay is expressed in the same time unit in which the counter reading is expressed. Thus, if an edge node is time-locked to a core node and the edge node transmits a pulse to the core node when the edge node time-counter reading is τ, the pulse should arrive at the core node when the core node time-counter reading is τ.
Time Division Multiplexing (TDM) and burst switching are two systems of time sharing. At an edge node using TDM, data is organized in a time-slotted frame of a predefined duration. Data traffic from a source node to a sink node may be allocated at least one time slot. In burst switching, data packets are aggregated, at an edge node, into bursts, where the bursts are generally of different duration. Each burst is switched at the core node towards the destination sink node of the burst. At the sink node, the burst may be disassembled into constituent data packets. Systems of time sharing can be exploited to increase the nodal degree and, by so doing, reduce the network diameter. The application of TDM in an optical-core network is described in Applicant's U.S. patent application Ser. No. 09/960,959, filed on Sep. 25, 2001 and titled “Switched Channel-Band Network,” the specification of which is incorporated herein by reference.
This division of capacity of optical links, provided by these schemes and others, allows a data stream, transmitted from a source node, to specify a level of service required while being transferred to a sink node, perhaps via one or more core nodes. It is then a necessity of the source node to have a service-quality control system to receive the requirement specified for the data stream and provide, if possible, the specified requirement.
The present-day Internet employs routers that were not built with the flexibility required to incorporate service-quality control in a scalable network. Additionally, the introduction of new services and new capabilities in the current Internet requires expensive patchwork and may result in a complex, engineering-intensive, difficult-to-control network.
Clearly, there exists a need for an edge node that incorporates service-quality control capabilities, and enables the construction of agile networks, i.e., networks that can efficiently accommodate wide variation of the spatial and temporal distribution of traffic, and that can transfer data streams having widely varied requirement and adapt to provide various levels of service.
With diverse transfer rate requirements, which may vary from a few Kb/s to a few Gb/s, there is a need for an edge node that is capable of efficiently handling extremes in transfer rate and distribution of traffic, and adaptively modifying path capacities without unmanageable control complexity.
There is also a need for an edge node that provides data-rate control and can work with an optical core node